Konami has just issued a statement on the Metal Gear Online site:
…we regret to inform you that due to extraordinary levels of traffic from all around the globe, we have been unable to run the service to the level we expected. As a temporary measure, we will be deferring the start of the US public beta test until we are able to stabilize our server. We would like to apologize for all inconveniences as well as confusion caused.
As a result, the beta that would take place today has been postponed, with an announcement on a new date expected on Tuesday. To compensate for this, the beta will be run for another week, and is now due to end on the 11th of May, rather than the 6th. I think I just heard a bunch of fans crying out at the same time.